Electric welders are employed in a variety of field applications, in which electric power is applied to a gap in a welding circuit between a workpiece to be welded and an electrode. One type of welding process is known as tungsten inert gas (TIG) welding, wherein heat is generated from an electric arc maintained between a non-consumable tungsten electrode and a part or workpiece being welded. Additional filler metal may, but need not, be employed, such as a separate filler metal wire, where additional material is desired. In TIG welding operations, a shield of inert gas, typically argon protects the melt puddle, electrode, and the optional filler rod from the ambient atmosphere, in order to prevent rapid oxidation of the weld and surrounding metal. TIG welder power supplies may be AC, DC, or combinations thereof, as determined according to the type of metal to be welded. DC welding is often used to weld stainless steel and mild and low alloy steels, whereas AC is typically used to weld aluminum. In AC welding, surface oxidation is removed in the half power cycle where the electrode is positive, and hence this is referred to as the “cleaning” half-cycle, while the negative half-cycle is referred to as the “penetration” half-cycle. The welding voltages and currents provided during the penetration and cleaning half-cycles are typically not equal, for instance, wherein more energy is applied by the welder during penetration than in cleaning. TIG welders, both DC and AC types, commonly include arc starting systems providing high frequency power to the welding circuit during arc initiation, which may be deactivated once the arc is established.
In DC TIG welding, currents are often provided to the welding circuit using a single phase SCR rectifier, wherein the welding current is adjustable by varying the phase firing angle of one or more SCR devices in an SCR network in an output rectifier. In industrial DC welding applications, currents as high as 200-300 amperes are common. However, in some applications, the same industrial DC welders are required to operate at much lower current levels, for example, such as 5-10 amps or less used in welding thin aluminum workpieces. In such a situation, the SCR is only actuated for a short time near the end of the positive power source half-cycle. As a result, arc stability at such low current levels suffers. An auxiliary or background supply may be employed at such low current levels in order to create a fixed minimum current, in conjunction with the phase controlled rectifier. In this implementation, however, problems have been found in starting or establishing the arc at such low current levels. Samodell U.S. Pat. No. 6,388,232 assigned to the assignee of the present application addresses the above shortcomings with respect to DC welders being started and operated at current levels of 5-10 amps or less.
In AC welding applications, alternating currents of 200-400 amperes are often supplied to the welding circuit via SCR controlled square wave supplies, having separate SCRs for connecting the welding electrode with positive and negative voltages. In such AC welders, the SCRs are gated or activated by gating signals during the positive or negative half-cycles of an AC supply source, where the portion of the supply half-cycle in which the SCR is gated determines the amplitude of the AC currents in the welding circuit. Thus, this type of AC welder may also employ phase firing angle control of SCR gating signals in order to provide adjustable AC welding currents. As with DC welders, AC welding equipment often is needed to operate at low current levels for certain applications, and much higher currents for other applications. Difficulties arise in starting and operating such equipment at the low end of the current range, for example, wherein operating levels below about 15-20 amps are desired.
Conventional AC and AC/DC welders often experience erratic operation at such low currents, wherein arc “popping” and “dancing high frequency” conditions are found. For example, a constant current (CC) square wave TIG (SWT) welder designed to operate at up to 200-400 amps controls the amount of AC current delivered to the welding circuit by varying the time that the control SCRs are in the conducting state using phase angle firing signals applied to the SCR control gates. Operation at low current levels requires the control signal to the SCR to be asserted very late in the power cycle, and for a very short time. As the “on-time” for the SCR is reduced, a point is reached at which the short current pulse cannot be sustained by the output current choke in the welder to maintain continuous current flow at high enough output voltage until the following half-cycle gate firing. In this situation, the low current “pops” out, and the arc stabilizing high frequency system is activated to maintain the TIG arc ionization, thereby causing high frequency arc “dancing”.
Heretofore, no AC or AC/DC SCR driven welding equipment has been developed which can support both high and low current levels without the above mentioned difficulties in starting and operating at low current levels. Thus, there remains a need for improved apparatus and methodologies for starting and operating AC and AC/DC welders at low current levels, by which the above and other difficulties may be avoided or mitigated.